Pure titanium or titanium alloys (hereafter may be referred to simply as titanium alloys) have excellent corrosion resistance in various corrosive environments where chlorides are present such as in seawater, and are heavily required in chemical plants or seawater desalination plants. However, titanium has a great affinity for hydrogen, and depending on the environment, it may therefore absorb a large amount of hydrogen. For example, if a titanium alloy is used for heat exchanger tubes in a seawater desalination plant, cathodic protection (cathode anti-corrosion) is given to prevent corrosion of steel materials in contact with the titanium alloy, but if this is done, the electrical potential of the members foi flied by the titanium alloy falls below the hydrogen generation potential, and the generated hydrogen is absorbed by the titanium alloy materials.
Titanium alloys easily absorb hydrogen in the aforesaid heat exchanger tubes, non-oxidizing acid solutions, hydrogen sulfide atmospheres such as those found in petroleum refineries, high-temperature steam environments such as the turbine blades of power generating stations and the high temperature gases of chemical plants.
Also, when titanium alloy materials come in contact with steel parts, and hydrogen is generated due to corrosion of the steel parts, the titanium alloy materials absorb this hydrogen and become brittle. When titanium alloy absorbs hydrogen, brittle hydrides are formed inside the titanium alloy, and if the amount of these hydrides is large, the member formed by this titanium alloy shatters even if a small external force less than the design stress acts on the member (hydrogen embrittlement fracture).
Due to the problem of embrittlement resulting from hydrogen absorption, the use of titanium alloy as a structural material is prohibited in environments where such hydrogen absorption may occur.
An example of a technique to prevent embrittlement of titanium alloy is for example to suppress hydrogen absorption by exposing the titanium alloy to atmospheric oxidation, as disclosed in the Journal of the Japan Seawater Academy No. 44, Vol. 3, or Anti-Corrosion Technology Vol. 28, p. 490 (1979). Specifically, when an oxide film is formed on the titanium alloy surface due to atmospheric oxidation, this oxide film blocks diffusion of hydrogen and thus suppresses infiltration of hydrogen into the alloy from the environment.
Further, in Japanese Patent No. 2824174 or Japanese Patent Application Laid-Open (JP-A) No. 07-3364, the infiltration of hydrogen is suppressed by making the surface coverage of titanium carbide, titanium nitride or titanium carbide/nitride equal to 1.0% or less. Specifically, titanium carbide, titanium nitride or titanium carbide/nitride is always formed during manufacturing processes such as rolling or annealing. The technique disclosed in Japanese Patent No. 2824174 describes the suppression of hydrogen absorption by reducing the amount of titanium carbide/nitride which would increase the hydrogen absorption rate of titanium alloy.
If an oxide film which blocks hydrogen diffusion is formed on the surface of a titanium alloy member subjected to atmospheric oxidation as described above, absorption of hydrogen by the titanium can be suppressed to some extent. However, when structural materials are used, it is difficult to avoid contact and shocks with other materials during construction work, so the atmospheric oxide film formed on the surface of the titanium alloy material becomes scratched or peels off. If this type of scratching or peeling occurs, this part is easily infiltrated by hydrogen, so compared to a titanium alloy material having an ideal atmospheric oxidation film formed in the laboratory, the hydrogen absorption suppression effect in actual materials is less.
The hydrogen absorption of titanium alloy can be suppressed to some extent also by reducing the surface coverage amount of titanium carbide/nitride. However, titanium alloy itself has a large affinity for hydrogen, so even if the surface amount of titanium carbide/nitride which accelerates hydrogen absorption is reduced, a satisfactory hydrogen absorption suppression effect cannot be obtained. Moreover, since titanium has a large affinity for carbon and nitrogen, even if the surface amount of titanium carbide/nitride formed in the manufacturing step is sufficiently removed, titanium carbide/nitride may be subsequently formed which increases the hydrogen absorption amount.
At the same time, when titanium alloy is used as a structural material for heat exchanger tubes or chemical equipment parts, cold working properties of identical level to that of JIS 2 pure titanium are required.
It is therefore an object of this invention, which was conceived in view of the above problems, to provide a titanium alloy material which could be used without risk of embrittlement fracture in environments where hydrogen is easily absorbed, and which has the same cold working properties as those of pure titanium.